An arrangement of the incipiently mentioned type is known from electrically driven compressors, which are often designed as compact modules and sometimes enclosed in a common casing, which is preferably made gas tight. These modules are capable to compress for example toxic gases or can be operated in explosive environments or even under water due to their capability to be designed without seals. These sealess compressors receive a process fluid through a suction line and deliver it through a discharge line at a higher pressure level. Since the borderline between pumps and compressors is difficult to draw, to both types of machines is referred hereinafter as pumps.
One feature of the sealess design is that substantially all components in the common gas tight casing are surrounded by the process fluid. This involves the disadvantage that all these components must be designed to withstand the eventually chemically aggressive process medium. One advantage is the possibility to use the process fluid to cool components of the arrangement without providing a dedicated cooling system. A cooling of the electric motor's stator and of bearings for the rotor, for example magnetic bearings can be done with the process fluid.
However, the process fluid flowing through the gap between the rotor and the stator of the electric motor and cooling the stator and the rotor at least partially can lead to rotor dynamic instabilities due to certain flow phenomena. Process fluid entering the gap is accelerated into the direction of rotation of the rotor circumferentially. If the rotor is displaced with respect to the rotation axis the gap is narrowed radially and the process fluid has to accelerate according to continuity. Following the Bernoulli's law, pressure decreases, where acceleration occurs, which amplifies the displacement of the rotor and maybe also sets off parts of the stator, which are moved to narrow the gap further. If critical parameters are exceeded the stator and the rotor might get in contact and can be damaged.